Power conversion circuits such as DC (direct current) power supplies and electronic ballasts frequently use a rectifier circuit to convert the incoming AC (alternating current) line power into DC power, which is then stored in a large electrolytic storage capacitor as a stable source of DC power for the system. The disadvantage of such arrangements is that the incoming line current is taken in the form of a large spike each time the power line voltage rises above the voltage of the storage capacitor. These current spikes are not proportional to the line voltage in the manner preferred by electric utility companies, and are characterized as having a poor power factor. The ideal situation is that the current drawn from the power line should be directly proportional to the voltage and in phase with it, in which case the power factor is said to be unity, i.e., there is exact correspondence in wave shape between the incoming voltage and the current which is drawn.
Many schemes for causing the current drawn from the power line to be sinusoidal have been described. One particular idea is to take some power from the output of the system and use it to pull current in from the AC power line with a sinusoidal waveform. One example of such a scheme is that described by Steigerwald in U.S. Pat. No. 5,113,337 for DC power supplies. Another analogous scheme for electronic ballasts was described by Mere in U.S. Pat. No. 5,134,556. These circuits are characterized by the presence of a charge pump. In these charge pumps, high frequency AC voltages at the output of the system are used to drive an arrangement of capacitors and diodes so that charge is "pumped" out of the AC power line and into the storage capacitor. With good control of the pumping voltage and correct sizing of the pumping capacitor, power factors of 0.999 may be achieved, implying that the current drawn from the power line is almost purely sinusoidal when the voltage is sinusoidal. These techniques are referred to as "charge pump power factor correction".
Although charge pump power factor correction is an effective and economical means of achieving power factor correction, all the schemes described have in common the following disadvantage: When the output load is removed, the charge pump will continue to operate so that the system will continue to extract energy from the power line even though no energy is being drawn from the output. Unless precautions are taken to avert this situation, the system will continue to absorb energy until it self-destructs through failure of some component. The methods by which the system is protected must be simple and inexpensive.